Just to give you some food for thought, I just talked to the four fellows listed at the sticky page here in the Horn section who do horn repairs. Each and every one of them was a living bundle of information regarding horns and what to do with them. I feel it would be worth your time to make those four calls and speak to each with any questions at all, as I found them well worth talking to and their expertise is amazing.
My own horn is a reproduction and some do not work on those at all. Believe me, they all have more work than one can imagine lined up for themselves with national meets and all going on in the future. I'd post their email addys, but I am hesitant to do that without their permission.
HORNS
1. No known website: Model A Horns (231) 577-1375 (Michigan)
2. No known website: Bill Randall: (315) 336-8841
3. No known website: Neil Stegall: (601) 469-0878
4. No known website: Jim Diehl: (231) 577-1375
None answered my initial phone call, but I left a phone message and they each returned my call quickly and were not hesitant at all to discuss my issues. I sent photos to some through their email and they knew exactly what the issue was by looking the photos over.
Good luck on your fix.